More than 100,000 New Zealand children a week are exposed to second-hand smoke in cars, a new study has found.

The findings, published in the New Zealand Medical Journal, have renewed calls for a ban on smoking in cars.

Co-author Ben Healey, from Otago University, said it was likely even more children were being exposed to cigarette smoke in cars.

"We've been deliberately conservative. While smoking in cars with children continues to be a problem, there are things that can be done about it."

Unlike adults, children were often unable to leave smoke-filled cars, or were uncomfortable asking an adult not to smoke. A law preventing smoking in cars would help people quit the habit, he said.

In 2010 a study found second-hand smoke killed more than 600,000 people worldwide every year.

The report published last week used data from the national-level annual Ash survey of year 10 students from 2006-2012.

School pupils were asked whether others had smoked around them in a car or van in the past week. Students who reported exposure on at least one day in the past week were classified as exposed to second-hand smoke.

Ash director Ben Youdan said it wanted smoking in cars banned as soon as possible.

"This is an area where legislation can make a difference to public health outcomes."

Associate Health Minister Tariana Turia last year said the Government was considering fines for smoking with children in a vehicle, banning smoking in parks and clamping down on duty-free tobacco limits.